1. The Field of the Invention
The present invention relates to the manufacture of semiconductor devices. More particularly, the present invention is directed to novel processes for use of germanium as an antireflective coating in active area and gate lithography steps.
2. The Relevant Technology
The need for increased miniaturization in integrated circuit semiconductor devices is well known. As feature sizes decrease, the need for more precise control of photolithography, often a limiting process, becomes acute. Better control of the line width produced by photolithography enables more aggressive circuit design for faster, higher performance circuits.
The line width produced by a photolithography process varies with many factors, including the amount of energy absorbed by the photoresist during exposure. For example, with positive photoresists, increased exposure energy decreases line width while decreased exposure energy increases line width. Precise control of line width thus requires precise control of exposure energies.
Control of exposure energy can be complicated by the varying reflectivity of the layers immediately below the photoresist. Photoresist over high reflectivity areas will be overexposed compared to photoresist over low reflectivity areas.
Nitride layers in particular vary significantly in reflectivity with varying thickness. The reflectivity of a nitride layer varies periodically with its thickness. At a minimum or maximum in the reflectivity function, reflectivity changes only slowly for every unit change of nitride thickness. Under current processing techniques, to avoid significant variation in exposure energy over the surface of a wafer, deposited nitride layer thickness is carefully controlled over the entire wafer surface to be as near as possible to a minimum or maximum of the periodic reflectivity function. But precise thickness control of nitride growth is difficult, particularly at the relatively thick 2000 Angstroms thickness that is employed in a typical active area stack, in which the required tolerance is approximately only xc2x150 Angstroms. Antireflective coatings (ARCs) can substantially reduce the reflected energy during photolithography, reducing the need for critical control of the thickness of underlying layers.
ARCs can also assist in avoiding standing waves in the photoresist during exposure. The presence of standing waves in the photoresist are problematic in that the reflected waves cause interference with the incoming wave and cause the intensity of the light to vary periodically in a direction normal to the photoresist. Standing waves cause variations in the development rate along edges of the photoresist, and degrade the image resolution. Further, standing waves can cause both necking and notching in the patterned area. ARCs helpful to reduce standing waves are typically a 130 nm thick polymer which has a high absorbance at the exposure wavelength so as to considerably reduce interference due to reflectance from the substrate.
Despite the forgoing benefits of using ARCs, conventional ARCs used in semiconductor processing are incompatible with the processes used in formation of the active area. The presence of an ARC complicates the task of etching. A layer of metal ARC, such as TiW or TiN, can contaminate the silicon substrate itself. Titanium nitride as an ARC produces mobile ionic contaminates in the active and isolation regions. Polysilicon as an ARC is not easily removed before field oxidation and results in unwanted oxide growth at the polysilicon locations during field oxidation, which oxide growth is not easily removed without removing or undesirably reducing the desired field oxide thickness. Accordingly, there is a need in the art to relieve the critical thickness control requirements of nitride layers in active area stacks.
An object of the present invention is to relieve the critical thickness control requirements of active area stack nitride layers.
Another object of the present invention is to provide an antireflective coating suitable for use in active area lithography.
Another object of the present invention is to provide an antireflective coating suitable for use in gate area lithography.
Another object of the present invention is improve line width control in active area photolithography.
Another object of the present invention is to improve line width control in gate area photolithography.
Another object of the present invention is to enable tighter, more aggressive circuit design for improved circuit speed and performance.
Another object of the present invention is to more easily remove photoresist.
In accordance with the present invention, a thin layer of germanium is deposited by sputtering, CVD, or other appropriate process over the top layer of a conventional gate or active area stack, such as a thick silicon nitride layer. The germanium layer is then covered with photoresist. Subsequent masking steps are then performed in a conventional manner.
The optical properties of germanium make it favorable as an ARC in that it has a high reflectivity of about 2.579 and a permitivity of about 4.07. Germanium has good compatibility with conventional active area and gate processing. Germanium provides good antireflective properties, is easily removed during subsequent processing, and does not contaminate the wafer. The antireflective properties of the germanium layer allow for much greater variation in the thickness of the underlying nitride layer, providing better line width control and with it the potential for decreased feature tolerance so as to allow for more aggressive circuit design. Fabrication efficiency is also increased due to the inherently better yield potential of improved line width control, and due to less rework of wafers having out-of-specification nitride layer thickness. The germanium layer also facilitates the complete removal of the photoresists, since the germanium layer upon which the photoresist is deposited is itself completely removed.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.